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The Black Death, which claimed an estimated 75-100 million lives, was one of the most devastating events that is still well known today.

What conditions would allow the plague to spread and kill on a larger or equal scale as the Black Death now or in the foreseeable future?

I am looking for natural causes, so bio-engineering a super plague is out.

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    $\begingroup$ Zika, swine flu, h5n1, hiv/aids, ebola. Sexual transmission, fluid contact, easy international travel, slow government reactions... $\endgroup$ – EveryBitHelps Aug 15 '16 at 12:33
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    $\begingroup$ The anti-vaccine movement is just enough. $\endgroup$ – makingthematrix Aug 17 '16 at 6:58
  • $\begingroup$ @makingthematrix Actually no, because even to unvaccinated people, a lot of the diseases we commonly encounter, particularly in the Western world, are not deadly. Vaccines are a very new-fangled invention; very useful, mind you, but not the only thing keeping a person with a random disease from dying a said-disease-related death. Also, unvaccinated people coming down even with a deadly disease that can be vaccinated against generally won't affect other, vaccinated people around them, to the limit that the vaccination is effective in the first place. $\endgroup$ – a CVn Aug 17 '16 at 9:29
  • $\begingroup$ Obligatory mention of relevant movie: imdb.com/title/tt1598778/combined $\endgroup$ – MichaelK Aug 17 '16 at 11:14
  • $\begingroup$ The place to start isn't Black Death but rather Spanish Flu $\endgroup$ – Separatrix Aug 17 '16 at 13:12
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For ultimate spread and mortality, you want something that has the following properties in combination:

  • High probability of infection when somebody is exposed to the disease (highly infectious)
  • High probability of transmission of the disease after being exposed, but before developing visible symptoms (highly infectious during the incubation period)
  • Long period between initial exposure and the disease manifesting itself (long incubation period)
  • Difficult (medically, politically or otherwise) to quarantine and cure infected (and potentially infected) individuals once the disease manifests itself (hat tip to Jay for this one)
  • High probability of death after the disease manifests itself (high mortality)

Also, a quick course of the disease once symptoms start to develop might make such a disease harder to prevent, because it provides less window of opportunity for treatment even if the disease is nominally treatable. It will also help if the early symptoms are similar to those of less dangerous diseases.

Ebola makes a poor epidemic fatal disease because while mortality is high, it quickly becomes clear that you are suffering from it, allowing for quarantine protocols to be initiated relatively early.

The common flu makes a poor epidemic fatal disease not least because with the exception of those who are for some reason already infirm, it isn't particularly deadly.

Thankfully to support what you want, presently, long-distance travel is the norm. A human who is infected by a disease in one part of the world can begin spreading it in a completely different part of the world within 12-24 hours under the right circumstances, making it difficult to establish proper quarantine of infected individuals. Under "ideal" circumstances, even the travel itself can easily transfer the disease to hundreds of individuals (a full airplane's worth). As Monica Cellio pointed out in a comment, an early outbreak at a place and time with a large worldwide attendance -- such as for example a major sports or religious event -- could exacerbate this.

The hard part is for the disease to be sufficiently deadly, yet not kill or incapacitate its hosts before the hosts have had time to sufficiently expose other potential hosts to the disease. If the disease fails the second criterion, outbreaks will be short and relatively geographically isolated. (There may be several related outbreaks in various parts of the world based on travel, but the disease is unlikely to significantly spread further before the outbreak dwindles down.)

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  • $\begingroup$ So if symptoms took say a month to develop and the outbreak was in some country where people go overseas everywhere, it will be very bad. Btw what did you mean by mobile Wikipedia. $\endgroup$ – Skye Aug 15 '16 at 13:51
  • $\begingroup$ @Sky The links in your question pointed at Wikipedia's mobile site (en.m.wikipedia.org), which gives everyone who follows them the mobile view, also on a desktop. $\endgroup$ – a CVn Aug 15 '16 at 13:52
  • $\begingroup$ SARS was a close call. Not quite infectious enough to go pandemic. People changed their behaviour ( masks, gloves, airport screening) and that reduced its infection ratio below one. With the next "new" vIrus we may not be so lucky. $\endgroup$ – nigel222 Aug 15 '16 at 14:04
  • $\begingroup$ And add in: brought to, or perhaps initial outbreak at, a large gathering with worldwide attendance -- Olympics, Hajj, Christmas mass at the Vatican... $\endgroup$ – Monica Cellio Aug 16 '16 at 1:01
  • $\begingroup$ @MonicaCellio Good point, added. $\endgroup$ – a CVn Aug 17 '16 at 9:22
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Ditto Michael Kjörling, I won't repeat his answer. I'd add:

  1. There is no known cure or circumstances make the cure difficult to distribute. If there was some disease fitting all MK's criteria, but it could be instantly cured by taking a pill that costs 5 cents to produce, I think the disease could be gotten under control quickly. If there is no cure or if the cure is hugely expensive or requires months of treatment, etc.

  2. Ignorance of what causes the disease or how it is spread. In the early days of a previously unknown disease, doctors are not going to know these things, and many people could die before they figure it out. For example, I read once that during the Black Plague, at one point in Britain the government concluded that the disease was spread by cats, and set about catching all the cats and killing them and burning the bodies. As the disease was really spread by rats, and cats tend to kill rats, this actually made the problem worse.

  3. Social or political factors prevent authorities from quarantining infected individuals, restricting travel, or taking other measures to slow the disease. At the height of the ebola crisis there was a case in the U.S. where a nurse travelled to Africa to help treat victims there, and on her return authorities wanted to quarantine her for a couple of weeks to insure she was not infected. She refused, threatened legal action when they tried to force her, got a bunch of media attention, and eventually the government backed down and let her circulate freely. It was an obvious case for someone to be quarantined: she had clearly been exposed many times over a long period. But the government didn't quarantine her for political and legal reasons.

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  • $\begingroup$ I borrowed (with attribution) your point on the disease being difficult to cure; hope you don't mind. $\endgroup$ – a CVn Aug 17 '16 at 9:25
  • $\begingroup$ Feel free. I pretty much borrowed your whole post with my first line. $\endgroup$ – Jay Aug 17 '16 at 17:30
  • $\begingroup$ About the black death; it has recently been theorized if not proven that rats did NOT in fact spread the black death. Rather, it was gerbils that housed the parasite which made the jump to humans. Go figure. $\endgroup$ – Isaac Kotlicky Aug 17 '16 at 22:34
  • $\begingroup$ @IsaacKotlicky you can find crackpot theories about anything - an enormous herd of gerbils scampering from China to Europe to spread the plague has got to be one of the more ridiculous I've heard. This got sparked by horrible reporting about a theory that Asian rodents (most likely marmots) were a persistent reservoir (of the bacteria - not parasite) between outbreaks. $\endgroup$ – pluckedkiwi Aug 18 '16 at 13:17
  • $\begingroup$ @pluckedkiwi can you source your statement? no one mentioned a herd of gerbils, just that researches have posited that rats may not have been the right rodent reservoir. $\endgroup$ – Isaac Kotlicky Aug 18 '16 at 13:29
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What you need is a species jump combined with a mutation. There currently is no pathogen that comes close to being able to inflict this kind of damage. Pathogens that deadly will have a strong tendency to burn out and go extinct.

When we see very lethal diseases, look around: they weren't human diseases. Smallpox jumped from probably rodents well after man walked the Earth, HIV jumped from monkeys about 100 years ago, the flu routinely jumps from birds and pigs. The plague is carried by rodents. Extreme killers come when a virus is much more dangerous to its new host than to its old one.

To reach Black Death levels you simply need a virus to jump and mutate, the result being something contagious but for which the lethal phase takes time to develop. Think of something that presents as the common cold but does something very nasty over time: it gets driven into hiding but eats away at the part of the body where it's hidden.

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  • $\begingroup$ What about something like the Tasmanian devil cancer? That stuff is scary... $\endgroup$ – Isaac Kotlicky Aug 17 '16 at 22:35
  • $\begingroup$ @IsaacKotlicky No, because blood-vector diseases won't get the spread needed for Black Death level epidemics. $\endgroup$ – Loren Pechtel Aug 17 '16 at 23:44
  • $\begingroup$ You said both jump and mutate - jump from devils to humans, mutate in that it becomes moisture/sweat/air borne rather than blood borne. $\endgroup$ – Isaac Kotlicky Aug 18 '16 at 13:14
  • $\begingroup$ @IsaacKotlicky The mutations upon species jump are normally much more minor than that. Going from blood borne to an air vector isn't easy!! $\endgroup$ – Loren Pechtel Aug 18 '16 at 20:56
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Conditions need for an epidemic the scale of the black death.

  1. Large cities with crap conditions.

  2. No sewer system in cities human waste often clogs up the streets especially in slums.

  3. Overall ignorance of how diseases spread and can be stopped.

  4. An out-of-control rodent population in cities.

  5. No cleanliness rituals or washing habits among the people.

  6. Trade spreading across countries and even continents.

You put all these together and you have the perfect breeding ground for a disastrous plague.

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  • $\begingroup$ 1: denser population just facilitates transmission by having more potential interactions, 'crap conditions' are irrelevant. 2: perhaps if fecal route is important to transmission - this reads more like you still adhere to the miasma theory of disease 3: then why do even medical professionals get ill? Why do even well-educated groups still get the flu? ignorance does not explain much 4: only if rodents are the transmission vector or disease reservoir, otherwise completely irrelevant 5: again depends on the transmission method 6: more travel is more contact with more people - share the love $\endgroup$ – pluckedkiwi Aug 18 '16 at 13:27
  • $\begingroup$ 1. Cramped conditions means more human interaction human interaction with more chance interacting with a human who has the plague. $\endgroup$ – Bryan McClure Aug 19 '16 at 13:38
  • $\begingroup$ @pluckedkiwi 2. As you also stated fecal route is important to the spreed of different types of sickness. In fact if you look back in history the absence of Plumbing has been the cause of many outbreaks of diseases throughout the Middle Ages $\endgroup$ – Bryan McClure Aug 19 '16 at 13:41
  • $\begingroup$ @pluckedkiwi 3. Overall ignorance means just that overall ignorance this means even the "educated" don't really understand diseases spread. Or worse they have false information on how it spreads. Again picture the doctors in the Middle Ages. They knew nothing of what causes disease, and very little of how diseases spread. $\endgroup$ – Bryan McClure Aug 19 '16 at 13:44
  • $\begingroup$ @pluckedkiwi rodents have connected to the spread of many diseases including the black plague. $\endgroup$ – Bryan McClure Aug 19 '16 at 13:46
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The most likely candidates for an extinction level event are Influenza (do research on H5N1 in China), and Ebola Zaire (research likelihood of cross mutation with Ebola Reston).

Influenza is a promiscuous virus, easily swapping gene segments with other strains. The Chinese variant (H5N1) has high mortality, but low morbidity (hard to catch, but if you catch it, you'll probably die). This strain is still primarily an avian strain, so you're only likely to catch it if you're already very sick (102-104 degree fever). Gene strands were swapped out in a level 4 biolab environment to demonstrate the ease with which it could happen in nature (reported in mass media: http://science.sciencemag.org/content/336/6088/1534.full ). Resultant strain had reduced mortality, but greatly increased morbidity. Unlike run of the mill influenza, this strain has a mortality spike in late teens/early twenties due to Cytokene Storm (healthy bodies fight the virus too hard, resulting in fatal damage to tissues). With a few minor mutations, this strain could easily replicate the H1N1 Spanish Influenza (also an avian strain) of 1918-1919, only with much higher death tolls. A recent article from Canada on this strain: http://wwwnc.cdc.gov/eid/article/20/9/14-0240_article

Ebola Zaire has high morbidity and high mortality, but the disease kills very quickly and it's usually clear when someone is infected. It's only transmitted by touch, so it easily contained. Ebola Reston, on the other hand, is easily transmitted via airborne vapor. It's not particularly deadly to humans, but could easily mutate with Ebola Zaire under a particular set of circumstances. The resultant strain would likely have a much higher morbidity, and lower mortality than a straight Zaire strain. Suggested reading: Richard Preston's The Hot Zone.

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The Black Death still exists, several thousand a year contract it, and occasionally a few people die. But these days we have decent sanitation, soap, a change of clothes, antibiotics, medicines, doctors, hospitals, and pest control.

It is however on a watchlist as a feared biological weapon released by aerosol as there is no vaccine and is simply a bacteria.

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Other answers have already covered many areas, so this'll just focus on one

Antibiotic Resistance

A perfect storm is brewing in the health sector where over use, over prescription and under-development have combined to lead to a situation where, in our lifetimes, antibiotics could essentially become useless against killer bacteria. To reach black death levels all that needs to happen is a new strain of an existing disease treated with Antibiotics to arise that is not only resistant, but essentially immune to current medicines. Indeed, this could be set very near future - a World Health Organization (WHO) report released April 2014 stated, "this serious threat is no longer a prediction for the future, it is happening right now in every region of the world and has the potential to affect anyone, of any age, in any country. Antibiotic resistance—when bacteria change so antibiotics no longer work in people who need them to treat infections—is now a major threat to public health.1

Potential "candidates" include the MRSA family - Methicillin-resistant Staphylococcus aureus - (so called because it is resistant to most broad spectrum antibiotics including methicillin 2)

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